{-# LANGUAGE DeriveAnyClass #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE DerivingStrategies #-}
module Voting.Protocol.Credential where

import Control.DeepSeq (NFData)
import Control.Monad (Monad(..), forM_, replicateM)
import Data.Bits
import Data.Bool
import Data.Char (Char)
import Data.Either (Either(..), either)
import Data.Eq (Eq(..))
import Data.Function (($))
import Data.Functor ((<$>))
import Data.Int (Int)
import Data.Maybe (maybe)
import Data.Ord (Ord(..))
import Data.Semigroup (Semigroup(..))
import Data.Text (Text)
import GHC.Generics (Generic)
import Numeric.Natural (Natural)
import Prelude (Integral(..), fromIntegral, div)
import Text.Show (Show(..))
import qualified Control.Monad.Trans.State.Strict as S
import qualified Crypto.KDF.PBKDF2 as Crypto
import qualified Data.Aeson as JSON
import qualified Data.Aeson.Types as JSON
import qualified Data.ByteArray as ByteArray
import qualified Data.ByteString as BS
import qualified Data.Char as Char
import qualified Data.List as List
import qualified Data.Text as Text
import qualified Data.Text.Encoding as Text
import qualified System.Random as Random

import Voting.Protocol.FFC

-- * Type 'Credential'
-- | A 'Credential' is a word of @('tokenLength'+1 '==' 15)@-characters
-- from a base alphabet of (@'tokenBase' '==' 58)@ characters:
-- "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz"
-- (beware the absence of "0", \"O", \"I", and "l").
-- The last character is a checksum.
-- The entropy is: @('tokenLength' * log 'tokenBase' / log 2) '==' 82.01… bits@.
newtype Credential = Credential Text
 deriving (Eq,Show,Generic)
 deriving newtype NFData
 deriving newtype JSON.ToJSON
instance JSON.FromJSON Credential where
	parseJSON json@(JSON.String s) =
		either (\err -> JSON.typeMismatch ("Credential: "<>show err) json) return $
		readCredential s
	parseJSON json = JSON.typeMismatch "Credential" json

credentialAlphabet :: [Char] -- TODO: make this an array
credentialAlphabet = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz"
tokenBase :: Int
tokenBase = List.length credentialAlphabet
tokenLength ::Int
tokenLength = 14

-- | @'randomCredential'@ generates a random 'Credential'.
randomCredential :: Monad m => Random.RandomGen r => S.StateT r m Credential
randomCredential = do
	rs <- replicateM tokenLength (randomR (fromIntegral tokenBase))
	let (tot, cs) = List.foldl' (\(acc,ds) d ->
			( acc * tokenBase + d
			, charOfDigit d : ds )
		 ) (zero::Int, []) rs
	let checksum = (neg tot + 53) `mod` 53 -- NOTE: why 53 and not 'tokenBase' ?
	return $ Credential $ Text.reverse $ Text.pack (charOfDigit checksum:cs)
	where
	charOfDigit = (credentialAlphabet List.!!)

-- | @'readCredential'@ reads and check the well-formedness of a 'Credential'
-- from raw 'Text'.
readCredential :: Text -> Either ErrorToken Credential
readCredential s
 | Text.length s /= tokenLength + 1 = Left ErrorToken_Length
 | otherwise = do
	tot <- Text.foldl'
	 (\acc c -> acc >>= \a -> ((a * tokenBase) +) <$> digitOfChar c)
	 (Right (zero::Int))
	 (Text.init s)
	checksum <- digitOfChar (Text.last s)
	if (tot + checksum) `mod` 53 == 0
	then Right (Credential s)
	else Left ErrorToken_Checksum
	where
	digitOfChar c =
		maybe (Left $ ErrorToken_BadChar c) Right $
		List.elemIndex c credentialAlphabet

-- ** Type 'ErrorToken'
data ErrorToken
 =   ErrorToken_BadChar Char.Char
 |   ErrorToken_Checksum
 |   ErrorToken_Length
 deriving (Eq,Show,Generic,NFData)

-- ** Type 'UUID'
newtype UUID = UUID Text
 deriving (Eq,Ord,Show,Generic)
 deriving anyclass (JSON.ToJSON)
 deriving newtype NFData
instance JSON.FromJSON UUID where
	parseJSON json@(JSON.String s) =
		either (\err -> JSON.typeMismatch ("UUID: "<>show err) json) return $
		readUUID s
	parseJSON json = JSON.typeMismatch "UUID" json

-- | @'randomUUID'@ generates a random 'UUID'.
randomUUID ::
 Monad m =>
 Random.RandomGen r =>
 S.StateT r m UUID
randomUUID = do
	rs <- replicateM tokenLength (randomR (fromIntegral tokenBase))
	return $ UUID $ Text.pack $ charOfDigit <$> rs
	where
	charOfDigit = (credentialAlphabet List.!!)

-- | @'readCredential'@ reads and check the well-formedness of a 'Credential'
-- from raw 'Text'.
readUUID :: Text -> Either ErrorToken UUID
readUUID s
 | Text.length s /= tokenLength = Left ErrorToken_Length
 | otherwise = do
	forM_ (Text.unpack s) digitOfChar
	return (UUID s)
	where
	digitOfChar c =
		maybe (Left $ ErrorToken_BadChar c) Right $
		List.elemIndex c credentialAlphabet

-- ** Type 'SecretKey'
type SecretKey = E

randomSecretKey :: Reifies c FFC => Monad m => RandomGen r => S.StateT r m (SecretKey c)
randomSecretKey = random

-- | @('credentialSecretKey' uuid cred)@ returns the 'SecretKey'
-- derived from given 'uuid' and 'cred'
-- using 'Crypto.fastPBKDF2_SHA256'.
credentialSecretKey :: Reifies c FFC => UUID -> Credential -> (SecretKey c)
credentialSecretKey (UUID uuid) (Credential cred) =
	fromNatural $
	BS.foldl' -- NOTE: interpret the SHA256 as a big-endian number.
	 (\acc b -> acc`shiftL`8 + fromIntegral b)
	 (0::Natural) $
	Crypto.fastPBKDF2_SHA256
	 Crypto.Parameters
	 { Crypto.iterCounts   = 1000
	 , Crypto.outputLength = 32 -- bytes, ie. 256 bits
	 }
	 (Text.encodeUtf8 cred)
	 (Text.encodeUtf8 uuid)

-- ** Type 'PublicKey'
type PublicKey = G

-- | @('publicKey' secKey)@ returns the 'PublicKey'
-- derived from given 'SecretKey' @secKey@.
publicKey :: Reifies c FFC => SecretKey c -> PublicKey c
publicKey = (groupGen ^)